As we enter the 21st century, race/ethnic disparities in infant mortality remain a poorly understood phenomenon. Inconclusiveness of results is partially attributable to use of century-old indicators of birth outcomes based on White standards which "may result, for other ethnic groups, in invalid risk assessments and the misidentification of infants in need of intervention services". A central task is to better document how birth weight and gestational age, independently and conjointly, differentially affect infant mortality risk across race/ethnic populations. Another general objective is to determine the extent to which there are race/ethnic and sex differences in the way the effects of other risk factors for on infant mortality work through, or outside of, the mediating effects of birth outcomes. Specifically, we propose to: (1) formulate models of infant mortality for 24 race/ethnic- and sex-specific populations using both main and conjoint effects of birth weight and gestational age in order to specify birth outcome optimums for infant survival and evaluate the "one-size-fits-all" assumption; (2) determine the extent to which it is possible to explain race/ethnic differentials in infant mortality based on progressive adjustment of models that contain race/ethnicity, sex, the continuous birth outcome measures, and a wide range of demographic, SES, behavioral and maternal health covariates; (3) estimate race/ethnic- and sex-specific models to determine the extent to which birth outcomes mediate the effects of demographic, SES, behavioral, maternal health, and other covariates on infant mortality risk; (4) utilize earlier NCHS linked files (1989-91) in conjunction with the most recent files (1995-98) to examine changes that have occurred in the relationships of interest over a time period in which a number of profound alterations in infant mortality occurred in the US; (5) inasmuch as infant mortality risk varies substantially by age of infant, incorporate the nonproportional effect of infant age into the analyses. Data will be drawn from the 1989-91 and 1995-98 NCHS Linked Birth/Infant Death cohort files which contain a sufficient number of cases to support the analysis.